Fast ion beams are of interest for various applications including production of radioactive isotopes, neutron production, radiography, fusion, and various forms of radiation therapy.
Beams of fast ions are typically produced in accelerators of various configurations such as cyclotrons or synchrotrons. Accelerators are relatively large and expensive machines that are costly to run and maintain. The development of lasers that are capable of providing extremely high intensities and electric fields has stimulated research in exposing matter to laser light electric fields to generate fast ions and interest in using lasers to provide relatively inexpensive fast ion sources.
U.S. Pat. No. 6,906,338 describes using laser pulses “having a pulse length between approximately 1 to 500 femtoseconds (fs)” focused to energy densities of between about 1018 to about 1023 Watts/cm2 (W/cm2) to produce a high flux of energetic ions such as protons that may be used for medical purposes. The pulses are directed to interact with targets of various designs and provide radiation components that “include different species of ions (e.g., protons), x-rays, electrons, remnants of the pulse 102, and different energy components (e.g., MeV, 10's MeV, and 100's MeV within a certain energy band or window)”. The targets may comprise a thin foil layer for absorbing pre-pulse energy of the pulses. A beam transport system allows ions, such as protons, produced in the target and having a predetermined beam emittance and energy to propagate to a “treatment field” for therapeutic applications. The patent describes targets that are concave on a side of the target downstream relative to a propagation direction of the laser pulses and may be formed having grooves, or comprising fibers, clusters, or foams. “The size of grooves, 402, fibers 404, clusters 406 or foams 408 may be designed to be shorter than the size of electron excursion in the pulse field (less than approximately 1 micron)”.